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Fool’s gold? Why blinded trials are not always best

BMJ 2020; 368 doi: (Published 21 January 2020) Cite this as: BMJ 2020;368:l6228

Linked Research

Impact of blinding on estimated treatment effects in randomised clinical trials

Linked Editorial

Blindsided: challenging the dogma of masking in clinical trials

  1. Rohan Anand,, doctoral research student1,
  2. John Norrie,, professor2,
  3. Judy M Bradley,, professor1,
  4. Danny F McAuley,, clinical professor1,
  5. Mike Clarke,, professor3
  1. 1Wellcome-Wolfson Institute for Experimental Medicine, School of Medicine, Dentistry, and Biomedical Sciences, Queen’s University Belfast, Belfast, UK
  2. 2Usher Institute of Population Health Sciences and Informatics, University of Edinburgh, Edinburgh, UK
  3. 3Northern Ireland Clinical Trials Unit and Methodology Hub, Centre for Public Health, Queen’s University Belfast, Belfast, UK
  1. Correspondence to: M Clarke m.clarke{at}

Blinding is intended to reduce bias but can make studies unnecessarily complex or lead to results that no longer address the clinical question, argue Rohan Anand and colleagues

The essence of blinding is withholding information about treatment assignment from people involved in the trial. Trials in which patients, clinicians, and researchers are blinded to the allocated intervention are usually regarded as the gold standard of clinical research and evidence.12 However, blinding’s illustrious reputation brings with it the danger that it is regarded as essential for a trial to be “good,” especially if users place an uncritical reliance on hierarchies of evidence in which blinded evaluations are near the top.3 Given that the number of new trials is increasing every year, with 25 000 registered since the start of 2019, we are concerned that a substantial amount of time, energy, and funding may be going into considering and implementing blinding without a sound rationale for it.4

Past, present, and future trials contain vast amounts of important data. If trials without blinding are inappropriately judged to be of lower quality than blinded trials then we may not be making best use of their data to improve healthcare, while blinded trials may be producing results that are more difficult to interpret than they need be. In this article, we seek to stimulate debate by challenging some of the prevailing beliefs on the benefits of blinding.

Purpose of blinding

The first recorded instance of blinding is from Paris in 1784.5 This was a study into an unknown, mystical force called mesmerism and the claims that patients could be healed simply by encountering a “mesmeriser” or an object that had been mesmerised. People believed to have been mesmerised would sometimes show hysterical behaviour, but the French Academy of Sciences was sceptical. It assembled a …

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